Thermally toughening glass

ABSTRACT

Hot glass is thermally toughened by generating a stream of closely-packed, aerated particles, and projecting that stream towards the glass. A plurality of such streams are projected from an array of nozzles, and the velocity of projection of each stream is sufficient to ensure that the integrity of the stream is preserved in its trajectory towards the glass.

This invention relates to the thermal toughening of glass, and moreespecially to methods and apparatus for thermally toughening glass inwhich hot glass is quenched with a particulate material.

Traditionally glass has been thermally toughened by directing cool airon to the surfaces of the heated glass. Attempts to increase the degreeof toughening achieved by increasing the rate of flow of the cooling airhave not always been commercially acceptable because of mechanicaldamage to the glass surfaces, producing optical defects which makes thetoughened glass sheets unacceptable for use as motor vehicle windows.

There have also been proposals for directing a quenching liquid againsthot glass surfaces in the form of jets or as an atomised spray of theliquid such as is disclosed in U.K. Pat. Nos. 441,017; 449,602 nowD274,527 and 449,864.

It has also been proposed to use as a toughening medium a suspension ofparticulate material in a gas flow. U.S. Pat. No. 3,423,198 relates tothe use of a gaseous suspension of a particulate organic polymerparticularly silicon rubber or polyfluorocarbon. U.S. Pat. No. 3,764,403describes the contacting of hot glass with a snow of sublimable carbondioxide.

It is a main object of the present invention to provide an improvedmethod and apparatus for the thermal toughening of glass in which aparticulate material is directed at the surfaces of glass in order toenhance the heat transfer away from those surfaces in the tougheningprocess.

According to the invention there is provided a method of thermallytoughening glass in which the hot glass is quenched with a particulatematerial, characterised by generating a stream of closely-packed,aerated particles, and projecting that stream towards the glass at avelocity which ensures that the integrity of the stream is preserved inits trajectory towards the glass.

Preferably the stream of particles has a voidage fraction in the range0.9 to 0.4. More particularly the voidage fraction may be in the range0.76 to 0.4. The component normal to the glass surface of the velocityof the stream of particles is preferably at least 1 m/s.

For smaller articles a single stream of particulate material may besufficient to obtain effective toughening of the whole article. For thequenching of larger glass articles, for example, a glass sheet to beused as a motor vehicle window, it is preferred to generate a pluralityof said streams of particles which are projected towards the surfaces ofthe glass.

Preferably during quenching the glass sheet is vertical and the streamsof particles are directed towards the surfaces of the sheet.

Alternatively the glass sheet may be supported horizontally and thestreams of particles are projected upwardly and downwardly towards thesurfaces of the sheet.

Another way of carrying out the invention is characterised by generatinga plurality of said streams of particles, and projecting those streamsinto a quenching, gas-fluidised bed of particulate material towards asurface of the glass which is immersed in the quenching bed. In thepreferred way of carrying out this method the glass sheet is suspendedvertically and is immersed in the quenching bed, and streams ofparticles are projected into the quenching bed towards both surfaces ofthe sheet.

Preferably the streams of aerated particles are generated by supplyingaerated particulate material to form the streams.

The streams of particles may be projected from arrays of nozzles whichcommunicate with a supply body of aerated particulate material.

In a preferred way of carrying out the method the supply body comprisesa falling supply of the particulate material including entrained gas,additional gas is supplied into the falling supply of particles adjacentthe nozzles, and the height of the supply body above the nozzles and thepressure of the additional gas supply are regulated to regulate thevelocity of projection of the streams from the nozzles towards the glassat a velocity which ensures that the integrity of each stream ispreserved in its trajectory towards the glass surface.

The pressure in the aerated material adjacent to the entrances to thenozzles may be regulated by maintaining a pressure above the surface ofthe supply body.

Preferably the streams of particles are projected from two verticalarrays of nozzles, each array of nozzles is supplied by a flow from afalling supply of aerated particulate material, and additional gas issupplied into the flows adjacent the arrays of nozzles.

This method may further comprise switching a gas supply to each flow ata plurality of locations which are spaced apart vertically relative toeach other adjacent the nozzles to initiate projection of the streams ofparticles towards the next glass sheet to be toughened.

The switching of gas supply to those locations may be selectively timed,and may begin at the lowermost location.

The invention also comprehends apparatus for thermally toughening glassby the method of the invention, characterised by means for containing asupply of aerated particulate material, means for generating from thatsupply a stream of closely-packed aerated-particles, means forprojecting that stream towards a surface of the glass, and means forregulating the velocity of projection of that stream.

The apparatus may comprise a container for a supply body of aeratedparticulate material, and an array of nozzles connected to the containerfor projecting streams of closely-packed aerated particles.

In a preferred embodiment the container is a supply duct which isconnected to a supply vesssel for containing a body of aeratedparticulate material, which supply vessel is positioned to provide aneffective head of pressure for supply of the particles, and porous tubesfor gas extraction and supply are located in the supply duct adjacent tothe entrances to the nozzles.

When toughening a suspended glass sheet the apparatus may comprise twosupply ducts each with a vertical array of nozzles, which arrays definebetween their outlet ends a vertical treatment space for the suspendedglass sheet, and two supply vessels respectively connected to the supplyducts.

Individual air slides may connect the supply vessels to the respectivesupply ducts to maintain the particulate material in an aerated state asit is supplied to the supply ducts.

The apparatus may further comprise a tank for collection of theparticulate material from the streams, collection chutes for particulatematerial mounted adjacent the tank to collect particulate material whichspills over the top edges of the tank, and recirculation conveyors whichlead from the collection chutes to the tops of the supply vessel orvessels to recirculate particulate material which spills from the tank.

In a modification the tank includes gas supply means at the bottom ofthe tank for creating a quenching, gas-fluidised bed in the tank, andthe tank is mounted on a lift operable to raise the tank to surround thearray or arrays of nozzles so that the streams may be projected into aquenching fluidised bed in the tank.

In another embodiment of the invention there may be a closed containerfor the supply body, with the array of nozzles connected to one side ofthe container, and gas supply means connected to the top of thecontainer to pressurise the space in the container above the supplybody.

When thermally toughening glass sheets there would be two such closedcontainers for two supply bodies of aerated particulate material eachcontainer having an array of nozzles, which arrays of nozzles arepositioned to define between them a treatment space for a hot glasssheet.

For the toughening of a horizontally supported glass sheet there may betwo supply ducts each with a horizontal array of nozzles, which arraysconstitute upper and lower arrays of nozzles which point towards eachother and define between them a horizontal treatment space for a glasssheet.

The invention also comprehends thermally toughened glass produced by themethod of the invention.

Some embodiments of the invention will now be described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 is a side elevation, partly in section, through one form ofapparatus according to the invention for thermally toughening glasssheets,

FIG. 2 is a front elevation, partly in section, of the apparatus of FIG.1,

FIG. 3 is a top plan view of the apparatus of FIGS. 1 and 2,

FIG. 4 is a diagrammatic vertical section through another embodiment ofapparatus for carrying out the invention,

FIG. 5 is a diagrammatic vertical section through another embodiment ofapparatus in accordance with the invention, for the thermal tougheningof a horizontally disposed glass sheet,

FIG. 6 is a view similar to FIG. 1, of a modification of the apparatusof FIG. 1 which includes a quenching, gas-fluidised bed, and

FIG. 7 is a side elevation, partly in section, of another form ofapparatus according to the invention.

Referring to FIGS. 1 to 3, a sheet of soda-lime-silica glass 1, which inthe embodiment illustrated is of rectangular shape but could be cut tothe shape of a windscreen, sidelight or rearlight of a motor vehicle, issuspended by tongs 2 in conventional manner by a suspension system 3which depends from a tong bar 4. The tong bar 4 is suspended by hoistcables 5 from a hoist system 6 of conventional kind which is mountedabove the roof of a vertical furnace of conventional construction whichis indicated generally by the reference 7. The hoist cables 5 runthrough sleeves 8 in the roof of the furnace 7 and vertical guide rails9 on which the tong bar 4 runs also extend through the furnace roof. Atthe bottom of the furnace 7 there is an open mouth 10 which can beclosed by hydraulically-operated doors 11. The furnace is mounted on aplatform 12 above which there is a frame structure 13 which carries thehoist system 6.

The platform 12 is mounted at the top of a vertical frame structure 14which extends upwardly from the floor 15.

Two vertical supply ducts 28 and 29 each have an array of nozzles, 30and 31 respectively, which project inwardly from the front faces of theducts 28 and 29. The ducts 28 and 29 are mounted on the frame structure14, and a treatment space for the glass sheet 1 is defined between theoutlet ends of the nozzles. The nozzles 30 and 31 of each array arearranged in a "domino-five" pattern extending from the vertical innerface of the respective supply ducts 28 and 29, which ducts are ofrectangular cross-section and extend vertically downwards from theoutlet ends of individual air slides 32 and 33 which lead from thebottoms of vertical supply vessels 34 and 35 containing columns ofparticulate material which is to be supplied in an aerated state to thenozzles 30 and 31.

The air slide 32 has a porous floor, indicated at 36, through which airis supplied from a plenum chamber 37. Compressed air is supplied to theplenum chamber 37 from a compressed air main 38, through a pressureregulator 39. Near the bottom of the supply vessel 34 air is suppliedthrough a porous sparge tube 40 to aerate and mobilise the particulatematerial in the supply vessel 34. The tube 40 is connected through apressure regulator 41 to the compressed air main 38. Similarlycompressed air from the main 38 is supplied from a plenum chamber 42through the porous floor 43 of the air slide 33, and to a porous spargetube 44 near the bottom of the supply vessel 35.

A recirculating conveyor system is provided, as will be described, tomaintain a supply of particulate material into the top of the supplyvessel 34 where the particles fall through a fine filter 45. Thedownfall of the particulate material through the vertical vesselentrains air from the top of the vessel, which entrained air, togetherwith the air from the slide 32, effectively aerates the particles in thevessel so that they are mobile and can flow downwardly like a fluid.This effect is enhanced by the provision of air at a regulated pressurethrough the sparge tube 40 at the bottom of the vessel 34, and throughthe porous floor 36 of the air slide 32 to provide a balanced system ofaeration to ensure fluidity of the particles which flow at theappropriate time into the top of the vertical supply duct 28.

The height of the usual surface level 46 of the column of particulatematerial in the vertical vessel 34 above the nozzles 30 provides, ineffect, a head of pressure in the supply of particles to the nozzles 30.With any particular nozzle array, this head of pressure contributes tothe control of the velocity with which streams of closely-packed aeratedparticles are projected from the nozzles 30 towards the glass to betoughened.

The oppositely positioned array of nozzles 31 is similarly supplied witha flow of aerated particulate material from the vertical duct 29 whichextends downwards from the air slide 33 which leads from the bottom ofthe supply vessel 35. There is a fine filter 47 at the top of the vessel35, and the usual surface level of the column of particulate material inthe vessel 35 is indicated at 48.

In each of the vertical supply ducts 28 and 29 there are a plurality ofporous gas supply tubes 49, for example of porous sintered metal. Thetubes 49 extend horizontally across the ducts behind and adjacent thenozzles and are equally spaced vertically at a plurality of locations ineach duct. The tubes 49 are adjustable horizontally towards and awayfrom the entrances to the nozzles. One end of each tube 49 is connected,outside the duct in which it is located, to a change-over valve 50, suchas a spool valve, which has a first inlet connected through a pressureregulator 51 to the compressed air main 38, and a second inlet connectedto a vacuum main 52. Operation of the spool valve is controlled by atimer 53.

In the embodiment illustrated there are six porous tubes 49, and thetimers 53 are under the control of an electronic sequence controller ofknown kind which controls a sequence of switching of gas supply from themain 38 to the tubes and of gas extraction from the tubes to the vacuummain 52.

When the tubes 49 are connected by the valves 50 to the compressed airsupply main 38, air which permeates from the tubes 49 constitutes asupply of additional air into the supply of aerated particles fallingdown the vertical ducts. Both the height of each supply bed, denoted bythe surface levels 46 and 48 of the columns of particulate material, andthe regulated pressure of the switched air supplies to the tubes 49 ineach duct 28 and 29, determine the pressure in the aerated particles atthe entrances to the nozzles. This determines the velocity at which thestreams of closely-packed aerated particles are projected from thenozzles 30 and 31 towards the surfaces of a glass sheet when it issuspended in the treatment space between the nozzles 30 and 31.

A porous tube 54 is located at the top of each supply duct 28 and 29,that is in the region of the entrance of the flow of particulatematerial into each duct. Each tube 54 is connected by a change-overspool valve 55 to the compressed air main 38 and the vacuum main 52. Thevalve 55 is controlled by a timer 56.

Associated with each of the supply vessels 34 and 35 there is a verticaldisc conveyor, 57 and 58 respectively. The conveyor 57 leads upwardlyfrom a hopper 59 to an outlet 60 which is positioned above the open topof the supply vessel 34. The hopper 59 is located beneath the dischargeend of an air slide 61 which is fixed at a slight angle to thehorizontal and is spaced from one side of a collection tank 62 toreceive particulate material which spills over one upper side edge 63 ofthe tank 62. The conveyor 58 leads upwardly from a hopper 64 to anoutlet 65 which is positioned above the top of the supply vessel 35. Thehopper 64 is located beneath the discharge end of an air slide 66 whichis also mounted at a slight angle as shown in FIG. 1, and receivesparticulate material from the other upper side edge 63 of the tank 62.

The hoppers 59 and 64 have coarse filters 67 and 68 through whichparticulate material falls from the discharge ends of the air slides 61and 66.

The cycle of operation for thermally toughening a glass sheet will nowbe described.

Initially there are regulated supplies of compressed air to the poroustubes 40 and 44 at the bottoms of the supply vessels 34 and 35, and tothe air slides 32 and 33. Supply bodies of aerated particulate materialare thereby maintained in a state of readiness in the vessels 34 and 35.Vacuum is switched to the porous tubes 49 and 54. The extraction of gasby the tubes 54 is effective to compact the particulate material at theregion of the outlets from the air slides 32 and 33 and impede flow ofparticulate material from the mobile bodies of aerated particulatematerial in the supply vessels. Extraction of gas through the tubes 49impedes any tendency of the particulate material to trickle through thenozzles 30 and 31.

The doors 11 at the bottom of the furnace are open and the tong bar 4 islowered by the hoist system so that the glass sheet 1 to be toughenedcan be suspended from the tongs.

The hoist system 6 is then operated to raise the tong bar to theposition in the furnace illustrated in FIGS. 1 and 2 and the furnacedoors 11 are closed. The glass remains in the furnace for sufficienttime to heat the glass sheet to a temperature near to its softeningpoint, for example in the range 620° to 680° C. by radiation fromelectric heaters in the walls of the furnace. When the glass sheet hasreached a desired temperature the doors at the bottom of the furnaceopen and the glass sheet is lowered rapidly at constant speed into thevertical treatment space between the nozzles 30 and 31. A dynamic brakemechanism in the hoist system 6 ensures rapid deceleration when theglass reaches its position indicated by dotted lines in FIGS. 1 and 2,between the nozzle arrays 30 and 31.

When there is a requirement to produce bent toughened glass sheets,bending dies may be positioned, in known manner, between the furnace andthe treatment space. The hot glass sheet is first lowered to a positionbetween the bending dies which are then advanced to close onto the glasssheet and bend it to shape. The dies are then retracted and the glass islowered into the treatment space.

Alternatively, or additionally, the suspension technique described inNo. GB-A-2 038 312 may be used either to assist bending when bendingdies are employed, or to effect bending of the suspended glass sheet.

When the glass sheet is stationary in the treatment space, the timers 56operate the change-over valves 55 which switch the tubes 54 from vacuumto compressed air supply. At the same time the timers 53 associated withthe lowermost tubes 49 switch the lowermost change-over valves 50 fromvacuum to compressed air supply and aeration of the stagnant particulatematerial at the bottom of the ducts 28 and 29 begins. The switchingsequence continues to switch rapidly the rest of the valves 50 to thecompressed air main 38.

There is instantaneous mobilisation of the particulate material in theducts 28 and 29, and because the flow of aerated particulate materialfrom the supply vessels 34 and 35 is no longer obturated by gasextraction through the tubes 54, the pressure head subsisting in thevessels 34 and 35 is immediately effective and the projection of streamsof closely-packed, aerated particles is initiated from the arrays ofnozzles towards the surfaces of the glass sheet.

The effective head of pressure, determined by the height of the fallingsupply of particles in the vertical vessels 34 and 35 and the pressureof air supplied through the porous tubes 49 determines the pressure inthe vertical supply ducts 28 and 29 just behind the nozzles arrays 30and 31. Streams of closely-packed aerated particles are thus projectedfrom the nozzles 30 and 31 towards the surfaces of the glass in thetreatment space, at a velocity which ensures that the integrity of eachstream is preserved in its trajectory towards the glass.

Excess particulate material spills over the side edges 63 and 67 of thetank 62 and falls down the chutes onto the air slides 61 and 66 fordelivery into the hoppers 59 and 64 and recirculation to the tops of thesupply vessels 34 and 35 by the conveyors 57 and 58. Soon after flow isinitiated replenishment of the particulate material in the supplyvessels 34 and 35 maintains the height of the supply beds at about thestatic surface levels indicated at 46 and 48.

At the end of a toughening period during which the glass sheet is cooledwell below its strain point, and toughening stresses are developing ascooling of the glass continues towards ambient temperature, the timercontrol causes the timers 53 and 56 to switch the valves 50 and 55 tovacuum thereby obturating the flow to the nozzles by compacting theparticulate material in the ducts 28 and 29 behind the nozzles and bycompacting the material in the region of the outlet from each of the airslides.

Mobility of the aerated supply bodies in the supply vessels ismaintained. When the extraction of gas through the tubes 54 hasestablished obturation of the flow of aerated material from the airslides, provision could be made to vent the tubes 49 to atmosphere ifthere is no tendency for the now stagnant material in the ducts 28 and29 to trickle out through the lower nozzles of the arrays.

One factor which has been found to influence the degree of tougheninginduced in the glass is the voidage fraction of each stream ofparticles, which is defined below, and is preferably in the range 0.9 to0.4. The effective pressure at the entrances to the nozzles, and hencethe velocity at which the streams of closely-packed, aerated particlesare ejected from the nozzles is such as to preserve the integrity ofeach stream in its trajectory towards the glass surface, with therequired voidage fraction.

The main controls are therefore the height of the supply beds of aeratedparticulate material, the pressure of gas released from the porous tubes49 in the vertical ducts 28 and 29, the time for which the jets areoperative, and the geometry of the nozzles and the nozzle arrays.

The amounts of air supplied to the individual tubes 49 as illustrated,or to pairs of these tubes, can be varied independently. This permitsindependent adjustment of the rate of flow of the particulate materialthrough parts of the nozzle arrays, so that uniformity of quenching canbe maintained.

In one arrangement of the apparatus for toughening glass sheets thelength of each of the nozzles in the arrays 30 and 31 was 30 mm and thenozzle bore was 3 mm. The nozzles were arranged in a "domino-five" arraywith a spacing between the nozzles of 20 mm×20 mm. Each nozzles arrayoccupied a space of 1010 mm×620 mm and there were 3200 nozzles in eacharray. The distance between the facing ends of the nozzles of the twoarrays was 115 mm. The height of the surface levels 46 and 48 ofparticulate material in the supply beds in the vertical vessels 34 and35 was about 2 m above the top of the nozzle arrays 30 and 31. Thetreatment space, 115 mm wide, between the ends of the nozzle issufficient to permit quenching of a flat glass sheet or a sheet whichhas been bent to the curved shape which is usual for a motor vehiclewindscreen.

Sheets of soda-lime-silica glass of overall dimensions 300 mm×300 mmwere toughened. Each glass sheet was heated to a pre-quenchingtemperature, for example 650° C., and then quenched in the streams ofparticles projected through the nozzles 30 and 31 into the treatmentspace.

Each stream was projected forwardly towards the glass surface at avelocity which ensured that the boundary of the stream did not becomediffuse and the integrity of the stream was preserved in its trajectorytowards the glass surface. Usually the streams impinged on the glassbefore they had curved downwardly to any substantial extent.

It was found to be preferable that each stream has a voidage fraction inthe range 0.9 to 0.4. The component normal to the glass surface of thevelocity of each stream of particles was at least 1 m/s.

The voidage fraction is an indication of the voidage within each streamof particles. For example, for each stream:

    Voidage fraction=(Vn-Vp)/Vn

where Vn=volume of a short length of the stream, and Vp=volume ofparticulate material in that short length of the stream.

The value of voidage fraction decreases as the degree of packing of theparticulate material increases, and for powdery material, falls to avalue in the region of 0.4 to 0.5 for static piles of powder or veryclosely packed bodies of powder which are in motion. At the other end ofthe range, as the voidage fraction increases above 0.9 towards thelimiting value of 1.0, which represents pure gas, there is only a minorproportion of powder present in the gas flow.

The streams of particulate material were directed at the glass surfacesfor a predetermined period sufficient to induce the required tougheningstresses in the glass, after which period the timers 53 actuate thechange-over valves 50, and the connection of the porous tubes 49 isswitched to the vacuum main 52. Gas extraction at the locations of thetubes 49 obturates the flow of particulate material through the nozzlesand the projection of particles from the nozzles towards the glass stopsquickly.

At the same time the timer 56 actuates the spool valve 55 to switch theporous tubes 54 to the vacuum main 52. The particulate material in theoutlet regions of the air slides 32 and 33 quickly impedes, and thenblocks, the flow of particulate material to the supply ducts 28 and 29.

The aerated particulate material in the air slides 32 and 33 and in thesupply vessels 34 and 35 is maintained in a mobile state in readinessfor the toughening of the next glass sheet.

At the end of a toughening operation the compressed air supplies to theair slides 32 and 33 and the porous tubes 40 and 44 may also be switchedoff, and the particulate material in the vessels 34 and 35 and the airslides 32 and 33 settles, but must be re-aerated before the nexttoughening operation.

Some examples of thermally toughening glass sheets by the method of theinvention and using the nozzle array just described are set out below.

EXAMPLE 1

The particulate material used was γ-alumina having the followingproperties:

Particle density=1.83 g/cm³

Particle size range=20 μm to 140 μm

Mean particle size=60 μm

A number of the sheets of glass of different thicknesses were heated to650° C. and then subjected to quenching with the streams of γ-aluminaunder the following conditions:

Pressure of air supply to supply tubes 49=0.172 MPa

Velocity of stream at exit from nozzles=1.88 m/s

Mass flow rate from each nozzle=10.1 g/s

Voidage fraction of each stream=0.602

The degree of toughening of glass sheets from 1.1 mm to 12 mm thick isrepresented in Table 1.

                  TABLE I                                                         ______________________________________                                                                Surface-                                              Glass       Central Tensile                                                                           Compressive                                           Thickness   Stress      Stress                                                (mm)        (MPa)       (MPa)                                                 ______________________________________                                        1.1          50          74                                                   2            63         108                                                   2.3          68         120                                                   3            80         148                                                   6           114         240                                                   8           120         266                                                   10          124         280                                                   12          128         286                                                   ______________________________________                                    

The central tensile stress was measured by a scattered light techniquein which a helium/neon laser beam was directed through an edge of theglass, and the retardation fringes measured in the first 20 mm to 30 mmof the glass surface to give a measure of the average central tensilestress in that area of the glass. The surface compressive stress wasmeasured using a differential surface refractometer.

Alteration of the pressure of the air supply to the supply tubes 49 hasan effect on the exit velocity of the streams of γ-alumina projectedfrom the nozzles and on the voidage fraction of each stream, asrepresented in Table II, which sets out results for the toughening ofglass sheets 2.3 mm and 3 mm thick which had been heated to apre-quenching temperature of 650° C.

                  TABLE II                                                        ______________________________________                                                                             Central                                                                       Tensile                                                                       Stress                                   Air Supply                                                                             Velocity at        Mass Flow                                                                              (MPa)                                    Pressure nozzle exit                                                                             Voidage  Rate     2.3  3                                   (MPa)    (m/s)     Fraction g/s      mm   mm                                  ______________________________________                                        0.035    1.12      0.714    4.34     52   56                                  0.103    1.35      0.533    8.74     66   75                                  0.172    1.88      0.602    10.1     68   80                                  0.276    2.3       0.626    11.73    72   84                                  ______________________________________                                    

These results indicate how an increase in the air supply pressure from0.035 MPa to 0.276 MPa results in an increase in the velocity of theparticle streams at the nozzle exits from 1.12 m/s to 2.3 m/s. Thevoidage fraction was within the range from 0.533 to 0.714. The mass flowrate of γ-alumina in each stream increases from 4.34 g/s to 11.73 g/s.The streams retained their integrity and impinged on the glass surfacebefore their trajectories had assumed any appreciable downwardcurvature, so that the component normal to the glass surface of thevelocity of impact of each stream on the glass was not appreciably lessthan the measured value at the nozzle exits. The normal component ispreferably at least 1 m/s, and in order to avoid damage to the glass itwas found preferable that the velocity component normal to the glasssurface should not exceed 5 m/s.

At a higher glass temperature, for example, 670° C. a somewhat higherdegree of toughening was produced. For example a central tensile stressof 87 MPa was induced in a 3 mm thick glass sheet when the air supplypressure to the tubes 45 was 0.276 MPa. Under the same conditions acentral tensile stress of 75 MPa was induced in a 2.3 mm thick sheet.

Care has to be taken to ensure that the glass surfaces are not damagedby too high a velocity of the particulate material impinging on thosesurfaces while they are hot and vulnerable. The upper limit of velocityof 5 m/s was found to be suitable.

A spacing between the nozzle ends down to about 50 mm to 60 mm may beemployed. As the spacing is increased the degree of toughening of theglass sheet is lessened, assuming that all other conditions remainconstant.

This was shown by varying the nozzle separation from 60 mm to 200 mmwhen toughening 2.3 mm thick sheets of glass heated to 650° C.m with anair supply pressure to the tubes 45 of 0.172 MPa. The results are inTable III.

                  TABLE III                                                       ______________________________________                                        Nozzle Separation                                                                           Central Tensile Stress                                          (mm)          (MPa)                                                           ______________________________________                                         60           90                                                               80           81                                                              120           68                                                              150           67                                                              200           66                                                              ______________________________________                                    

This indicated that variation of the nozzle spacing in the region fromabout 120 mm to about 60 mm gave another valuable way of varying thevelocity of the streams where they impinge on the glass, and thusvarying the stresses induced in the glass.

A nozzle separation of 200 mm is sufficient to accomodate from 80% to90% of the usual range of curved glass sheets for motor vehiclewindscreens, and 95% of usual glass sheets for vehicle rear and sidewindows.

EXAMPLE 2

Similar trials to those of Example 1 were carried out using aluminatrihydrate (Al₂ O₃.3H₂ O) having the following properties:

Particle density=2.45 g/cm³

Particle size range=20 μm to 160 μm

Mean particle size=86 μm

A number of sheets of glass of different thicknesses were heated to 650°C. and then quenched with streams of the alumina trihydrate under thefollowing conditions.

Pressure of air supply to supply tube 49=0.172 MPa

Velocity of stream at exit from nozzles=1.77 m/s

Mass Flow rate from each nozzle=10.38 g/s

Voidage fraction of each stream=0.68.

The degree of toughening of glass sheets from 1.1 mm to 12 mm thick isrepresented in Table IV

                  TABLE IV                                                        ______________________________________                                        Glass      Central Tensile                                                                           Surface Compressive                                    Thickness  Stress      Stress                                                 (mm)       (MPa)       (MPa)                                                  ______________________________________                                        1.1         53          79                                                    2           68         110                                                    2.3         72         122                                                    3           82         150                                                    6          126         259                                                    8          138         288                                                    10         140         300                                                    12         142         309                                                    ______________________________________                                    

It was again demonstrated how alteration of the pressure of the airsupply to the tubes 49 affects the exit velocity of the streamsprojected from the nozzles, the voidage fraction of the streams, and thedegree of toughening of the sheets. The results, with glass sheets 2 mm,2.3 mm, and 3 mm thick, heated to 650° C. are similar to those usingγ-alumina, are set out in Table V.

                  TABLE V                                                         ______________________________________                                        Air    Velocity          Mass                                                 Supply at nozzle         Flow  Central                                        Pressure                                                                             exit     Voidage  Rate  Tensile Stress (MPa)                           (MPa)  (m/s)    Fraction g/s   2.0 mm                                                                              2.3 mm                                                                              3 mm                               ______________________________________                                        0.035  1.13     0.736    5.65  46    54    58                                 0.103  1.51     0.66     9.35  60    68    78                                 0.172  1.78     0.683    10.38 68    72    82                                 0.276  2.51     0.729    12.44 72    76    85                                 ______________________________________                                    

These results show that when using alumina trihydrate an increase in thepressure of the air supply to the tubes 49 from 0.035 MPa to 0.276 MParesults in an increase in nozzle exit velocity from 1.13 m/s to 2.51m/s. The voidage fraction lies in the range 0.66 to 0.736. The mass flowrate of alumina trihydrate in each stream is increased from 5.65 g/s to12.44 g/s, and the streams had the same form as in Example 1.

At a higher glass temperature, for example 670° C., a higher centraltensile stress of 87 MPa was achieved in a 3 mm thick glass sheet whenthe air supply pressure was 0.276 MPa.

EXAMPLE 3

With the same nozzle array and dimensions a mixture of 95% by volume ofthe alumina trihydrate of Example 2 with 5% by volume of sodiumbicarbonate, was used for toughening sheets of glass 2.3 mm thick and ofoverall dimensions 300 mm×300 mm. The sodium bicarbonate had a meanparticle size of 70 μm and a material density of 2.6 g/cm³. Higherstresses were achieved than those achieved by quenching with aluminatrihydrate alone. The results obtained are summarised in Table VI.

                  TABLE VI                                                        ______________________________________                                                  Central Tensile Stress (MPa)                                        Air Supply Pressure                                                                       Glass Temp.                                                                              Glass Temp.                                                                              Glass Temp.                                 (MPa)       630° C.                                                                           650° C.                                                                           670° C.                              ______________________________________                                        0.035       49         59         63                                          0.103       70         78         81                                          0.172       74         84         87                                          0.276       76         86         89                                          ______________________________________                                    

Even higher stresses were produced in 3 mm thick glass under the sameconditions as shown in Table VII.

                  TABLE VII                                                       ______________________________________                                                  Central Tensile Stress (MPa)                                        Air Supply Pressure                                                                       Glass Temp.                                                                              Glass Temp.                                                                              Glass Temp.                                 (MPa)       630° C.                                                                           650° C.                                                                           670° C.                              ______________________________________                                        0.035       53         63         66                                          0.103       75         84         87                                          0.172       77         86         89                                          0.276       79         88         92                                          ______________________________________                                    

EXAMPLE 4

A similar nozzle array to that used for Examples 1 to 3, was employed,but the nozzle bore was 2 mm.

The same alumina trihydrate as in Example 2 was used.

Glass sheets 2.3 mm thick were heated to 650° C. and then quenched withstreams of the alumina trihydrate. The operating conditions and resultsachieved are set out in Table VIII.

                  TABLE VIII                                                      ______________________________________                                        Air    Velocity          Mass  Central                                                                              Surface                                 Supply at nozzle         Flow  Tensile                                                                              Compressive                             Pressure                                                                             exit     Voidage  Rate  Stress Stress                                  (MPa)  (m/s)    Fraction g/s   (MPa)  (MPa)                                   ______________________________________                                        0.103  1.48     0.52     5.37  71     120                                     0.137  1.78     0.483    7.1   73     123                                     0.276  2.17     0.53     7.86  78     132                                     ______________________________________                                    

EXAMPLE 5

With the same nozzle array as in Examples 1 to 3, the particulatematerial used for thermally toughening a glass sheet 2.3 mm thick was a"Fillite" powder which comprises hollow glass spheres derived frompulverised fuel ash from power station boilers, having the followingcharacteristics:

Material density=2.6 g/cm³

Particle density=0.38 g/cm³

Particle size range=15 μm to 200 μm

Mean particle size=80 μm

The air supply pressure to the supply tubes 45 was adjusted to producestreams of the "Fillite" having an exit velocity of 1.4 m/s from thenozzles and a voidage fraction of 0.76.

The 2.3 mm thick glass sheet was heated to 650° C. before quenching andthe central tensile stress in the toughened glass sheet was 58 MPa.

EXAMPLE 6

With the same nozzle array as in Examples 1 to 3, the particulatematerial used was 150 mesh zircon sand having the followingcharacteristics:

Particle density=5.6 g/cm³

Particle size range=30 μm to 160 μm

Mean particle size=110 μm

The results achieved when toughening glass sheets 2.3 mm thick aresummarised in Table IX.

                  TABLE IX                                                        ______________________________________                                        Air                       Mass                                                Supply Velocity at        Flow                                                Pressure                                                                             nozzle exit                                                                             Voidage  Rate  Central Tensile Stress                        (MPa)  (m/s)     Fraction g/s   (MPa)                                         ______________________________________                                        0.103  1.5       0.86     8.25  50                                            0.172  1.7       0.865    9.02  65                                            0.276  2.2       0.80     16.88 82                                            ______________________________________                                    

EXAMPLE 7

By varying the nozzle design without changing the air supply pressuresto the tubes 49, it was found that higher exit velocities could beachieved.

This was demonstrated by using the same alumina trihydrate as in Example2 projected from two vertical nozzle arrays.

In each array the nozzles were arranged in a "domino-five" array with aspacing between nozzles of 20 mm to 20 mm. The length of each nozzle was55 mm and the nozzle bore was 3 mm. Each array occupied a space of 1010mm×620 mm and the distance between the facing ends of the nozzles of thetwo arrays was 85 mm.

Sheets of glass 2.3 mm thick were heated to 630° C., 650° C. and 670° C.and were quenched by streams of alumina trihydrate projected from thisarray with air supply pressures of 0.103 MPa, 0.172 MPa and 0.276 MPawhich were used in the tests of Example 2.

The results obtained are set out in Table X.

                                      TABLE X                                     __________________________________________________________________________    Air Supply                                                                          Velocity at                                                                              Mass Flow                                                                           Central Tensile Stress (MPa)                           Pressure                                                                            nozzle exit                                                                         Voidage                                                                            Rate  Glass Temp.                                                                          Glass Temp.                                                                          Glass Temp.                              (MPa) (m/s) Fraction                                                                           g/s   630° C.                                                                       650° C.                                                                       670° C.                           __________________________________________________________________________    0.103 1.6   0.729                                                                              7.46  61     66     67                                       0.172 2.32  0.741                                                                              10.38 70     73     77                                       0.276 4     0.823                                                                              12.21 72     77     81                                       __________________________________________________________________________

In these examples streams of closely-packed aerated particles with avoidage fraction in the range 0.87 to 0.53 are effective.

A voidage fraction in the range 0.76 to 0.4 has been found to give goodresults.

Differential toughening effects, for example to produce vision zones ina glass sheet for incorporation in a windscreen, can be achieved byarranging the nozzles in each array according to the desired pattern ofregions of higher stress to be induced in the glass sheet which regionsof higher stress are interspersed amongst regions of lower tougheningstress through which there is adequate vision in the event of fractureof the sheet.

The suspended hot glass may be transported horizontally through thetreatment space between the vertical frames. In another way of operatingthe glass sheets to be toughened may be supported at an angle to thevertical, for example an angle of 45° to the vertical, and moved in ahorizontal path between arrays of nozzles through a treatment spacewhich is oriented at a similar angle to the vertical.

Some of the nozzles may be aimed inwardly so as to project streams ofparticles towards the edges of the glass sheet and enhance the stressingof the edges of the sheet. In another arrangement the nozzles inmarginal regions of the arrays may be directed inwardly to cause ageneralised flow towards the centre of the glass sheet being toughened.

Another embodiment of apparatus for carrying out the invention isillustrated in FIG. 4.

Two tanks 69 and 70 containing fluidised particulate material, have sidewalls 71 and 72 which are perforated. The arrays of nozzles 30 and 31extend from those side walls. The spacing between the nozzle ends is 110mm and the glass sheet 1 to be thermally toughened is lowered into thetreatment space between the ends of the nozzles.

Aerated particles are supplied to each of the nozzles 30 and 31 fromfluidised particulate material in the tanks 69 and 70.

A porous membrane 73 at the bottom of the tank 69 forms the roof of aplenum chamber 74 to which fluidising air is supplied through a supplyline 75. The top of the tank 69 is closed by a roof 76 which has aninlet port 77 connected to a filling duct 78 which includes a valve 79.Particulate material is filled into the tank 69 through the duct 78 whenthe valve 79 is open. An air duct 80 communicates with an aperture inthe roof 76. In the duct 80 there is a valve 81 by means of which thehead-space in the tank 69 can either be connected to a pressure line 82or can be vented through an exhaust line 83.

A further duct 84 is connected to an aperture in the roof 76 near to theside wall 71 of the tank 69. The duct 84 provides an outlet above a partof the fluidised bed in the tank 69 which is divided from the main partof the bed by a baffle 85 which extends downwardly from the roof 76. Thelower end of the baffle 85 is spaced above the porous floor 73 of thetank so as to provide a path, indicated by the arrow 86 for the flow offluidised particulate material, from the main part of the tank to thespace between the baffle 85 and the side wall 71 of the tank, whichsupplies aerated particles to the nozzles 30. Excess fluidised air isvented through the duct 84.

The same reference numerals are used for the roof structure with itsinlet and outlet ducts at the top of the identical tank 70.

At the bottom of the tank 70 there is a porous membrane 87 through whichfluidising air is supplied from a plenum chamber 88 which has its ownair supply 89. A flow of aerated particles is supplied from the tank 70beneath the bottom of the baffle 85 as indicated by the arrow 86 tosupply the nozzles 31. When an appropriate amount of the selectedparticulate material has been filled into both the tanks 69 and 70, thevalves 79 are closed and the valves 81 connect the pressure lines 82 tothe ducts 80 so that a pressure is maintained above the fluidised bedsin the tanks 69 and 70. The pressure of the supplies of fluidising airthrough the ducts 75 and 89 to the plenum chmabers 74 and 88 is suchthat the particulate material in the tanks 69 and 70 is in a suitablefluidised condition despite the pressure indicated by arrows 90 which ismaintained in the headspaces above the two fluidised beds.

By regulating the pressure of the supply of fluidised air through theducts 75 and 89 in relation to the pressures 90 maintained above thesurfaces of the fluidised supply beds, the pressure in the aeratedparticles which flow to the arrays of nozzles 30 and 31 is controlled toensure that streams of closely-packed aerated particles are projectedtowards the surfaces of the glass at a velocity which ensures that theintegrity of the streams are preserved in their trajectories towards theglass surface. The switching of the air supplies is controlled insimilar manner to that of the embodiment of FIGS. 1 to 3.

Particulate material supplied through the nozzles 30 and 31 is collectedand fed to a separate storage tank and in due course returned to theducts 78 of the tanks 69 and 70.

The use of the baffles 85 permits the level of fluidised particulatematerial in the tanks 69 and 70 to fall without detriment to thetoughening effect which is achieved since a constant pressure ismaintained in the headspaces above the surfaces of the fluidisedmaterial in the tanks 69 and 70. Venting of gas through the ducts 84helps to regulate the pressure in the aerated particles being fed to thenozzles.

FIG. 5 of the drawings, shows a further embodiment of the inventionsuitable for the thermal toughening of a horizontally supported glasssheet 91.

Horizontally disposed supply ducts 92 and 93 containing fluidisedparticulate material have upper and lower horizontal arrays of nozzles,30 and 31 respectively.

The nozzles 30 project downwardly from the lower face of the supply duct92 and the nozzles 31 project upwardly from the upper face of the supplyduct 93. A horizontal treatment space for a glass sheet 1 is definedbetween the ends of the nozzles.

A vertical supply vessel 94 connects with the upper supply duct 92through its upper face and a supply vessel 95 connects with the lowersupply duct 93 through one side. There are porous tubes 96 in each ofthe supply ducts 92 and 93.

Additional porous tubes 97 and 98 are fitted at the base of the supplyvessel 95, the tube 98 being connected in parallel with the tubes 96 ofthe supply duct 93.

Prior to the processing of a glass sheet, vacuum is switched to thetubes 96 in the supply ducts 92 and 93. Vacuum is also switched to thetube 98 at the base of the supply vessel 95.

By this means the particulate material in the supply ducts 92 and 93 isheld in a compacted non-aerated condition. Air is supplied continuouslyto the tube 97 at the base of the supply vessel 95 so that theparticulate material in the vessel 95 is kept aerated in a state ofreadiness.

A glass sheet 91 which has been heated to a prequenching temperature issupported on a frame 99 and moved into the horizontal treatment space.Air is then supplied to the tubes 96 in the upper supply duct 92 and tothe tubes 96 and the tube 98 in the lower supply duct 93.

Aeration of the particulate material in the supply ducts 92 and 93 issuch that the toughening effect of the particulate material which isprojected downwardly through the nozzles 30 onto the upper face of theglass sheet is substantially the same as the toughening effect of theparticulate material which is projected upwardly through the nozzles 31towards the lower face of the glass sheet.

FIG. 6 illustrates, in a view similar to FIG. 1, another way ofoperating the invention in which the supply ducts 28 and 29 becomeimmersed in a quenching, gas-fluidised bed of the particulate materialinto which the hot glass sheet 1 is lowered. The streams are projectedfrom the nozzles into the fluidised bed at a velocity which ensures thatthe integrity of each stream is preserved in its trajectory through thefluidised bed towards the glass.

The nozzle arrays 30 and 31 and the supply of fluidised particulatematerial is the same as described with reference to FIGS. 1 to 3.

Mounted on the floor 15, within the frame structure 14 is ascissors-lift table 100 surrounded by a bellows 101. The table 100 isindicated by chain-dotted lines in its lowered position. On the table100 there is a container 102 for a quenching, gas fluidised bed of thesame particulate material as is supplied to the nozzles 30 and 31. Thecontainer is of rectangular, horizontal cross-section and has an opentop. The floor of the container is formed by a porous membrane whoseposition is indicated by the reference 103. This porous membrane 103 isalso the roof of a plenum chambe which is indicated generally by thereference number 104.

The plenum chamber 104 is divided into three parts by partitions, therebeing a central part which has its own air supply and is located beneaththe treatment space; and two outer parts which have a common air supply.Air is supplied at a higher pressure to the central part of the plenumchamber than to the outer parts.

The porosity of the membrane 103 is such that there is a high pressuredrop in the air flow through the membrane. The pressure of the airsupply to the central part of the plenum chamber is such that thecentral part of the fluidised bed in the container 102 is in aquiescent, uniformly expanded state of particulate fluidisation. Theamount of the particulate material which is initially present in thecontainer 102 is such that when fluidising air is supplied to the plenumchamber 104 the level quiescent surface of the fluidised bed is abouthalf-way up the container.

Cooling tubes, not shown, may be mounted in the container near to itsside walls to maintain the fludised bed at a suitable quenchingtemperature, for example of the order of 60° C. to 80° C.

By operation of the scissors-lift table 100, the container 102 is raisedfrom its lowered position to the raised position illustrated in fulllines. The two vertical supply ducts 28 and 29 are immersed in thefluidised bed and the displacement of the fluidised material by theducts is such that the fluidised bed then fills the container and mayspill slightly over the top edge of the container.

The air slide 61 is spaced one side of the container 102 to receiveparticulate material which spills over the top edge of the containerinto two collection chutes 105. There are four chutes 105 fixed to thecontainer, which chutes together encircle the whole of the top edge ofthe container. The other two collection chutes 105 discharge on to theair slide 66. Each of the chutes leads downwardly to a throat 106 towhich a spout 107 is hinged. When the container 102 is being raised orlowered the spouts 107 are hinged upwardly, and when the container is inthe raised position the spouts are hinged downwardly to overlie the airslides 61 and 66.

The cycle of operation is similar to that described for the embodimentof FIGS. 1 to 3. After the furnace doors 11 have been closed and thesuspended glass sheet is heating in the furnace, the scissors-lift tableis operated to raised the container. The spouts 107 are hinged-upwardlyso that they clear the air slides 61 and 66. As soon as the table 100starts to rise the conveyors 57 and 58 are started. When the containeris in its raised position the air supplies to the plenum chamber 104 areswitched on.

The air supplied to the plenum chamber 104 fluidises the particulatematerial in the container 102 with particulate material in the treatmentspace between the arrays of nozzles in a quiescent uniformly expandedstate of particulate fluidisation.

The furnace doors 11 then open and the hot glass sheet is loweredrapidly at constant speed into the treatment space. Immediately afterthe lower edge of the glass sheet has passed downwardly through thehorizontal, quiescent upper surface of the fluidised particulatematerial, air is switched to the porous tubes 49, and to the air slides52 and 57. Aerated particulate material flows from the supply vessels 34and 35 to the nozzles at a pressure such that coherent streams of theparticulate material are projected towards the glass sheet through thequiescently fluidised material in the treatment space.

Particulate material spills over the upper edge of the container and isrecirculated to the supply vessels 34 and 35 to maintain the staticsurface levels of the supply fluidised beds.

The quiescent fluidised bed in the container 102 itself imparts abackground level of stress to the glass and the heat transfer from theglass surfaces is enhanced by the effect of the submerged streams fromthe nozzles which reach the glass surfaces and enhance localisedagitation of the particulate material at the glass surfaces and producea more uniform pattern of stressing of the glass than that produced bythe streams of particulate material alone.

FIG. 7 illustrates another apparatus according to the invention, forbending and toughening glass sheets.

The same reference numerals are used as in FIGS. 1 to 3 to indicate thesame or similar parts.

The furnace 7 is located at the bottom of the apparatus, and bendingdies 108 and 109 are mounted above the furnace mouth 10.

The supply ducts 28 and 29, with their nozzle arrays 30 and 31 are lowersections of vertical ducts whose upper sections constitute the supplyvessels 34 and 35. The air slides 32 and 33 of the embodiment of FIGS. 1to 3 are not necessary.

Aeration of the particulate material in each of the upper supply parts34 and 35 of the ducts is effected by two pairs of porous tubes 40. Onepair of tubes 40 is mounted about half-way up each of the upper section.The lower pair of tubes 40 is mounted near the bottom of the uppersection. Each pair of tubes 40 is connected through a pressure regulator41 to the compressed air main 38. The continuous supply of compressedair to the tubes 40 maintains the supply body of particulate material inthe upper sections in readiness in an aerated state.

At the top of each of the lower sections 28 and 29, just above thenozzle arrays 30 and 31 there is mounted a bank of three porous tubes 54which are connected in parallel to a change-over valve 55 which iscontrolled by a timer 56. One inlet to the valve 55 is connecteddirectly to the vacuum main 52. The other inlet to the valve 55 isconnected through a pressure regulator 114 to the compressed air main38.

In each of the lower sections 28 and 29 there are ten vertically spacedporous tubes 49 which are connected in pairs to change-over valves 50,which are controlled by timers 53, and have inlets connected directly tothe vacuum main 52 and inlets connected through pressure regulators 51to the compressed air main 38.

Operation is similar to that of the apparatus of FIGS. 1 to 3. Theswitching of vacuum to the banks of three porous tubes 54 in the outletregion from the upper supply sections 34 and 35 of the vertical ducts,serves to effect a positive compaction of the particulate material inthose regions above which the aerated supply bodies are supported untilflow is required.

The hot sheet 1 is, raised from the furnace to bending position betweenthe dies 108 and 109 which close on to the sheet. After the dies openthe bent sheet, which is still hot, is raised to the position shown inthe treatment space between the arrays of nozzles 30 and 31.

A powder collection chute 115 moves beneath the nozzle arrays, and thevalves 55 then switch compressed air to the tubes 54. This releases thesupply bodies of aerated particulate material in the upper sections 34and 35, and falling flow of material in the vertical ducts is initiatedto feed the streams projected from the nozzles as a result of sequentialswitching of compressed air to the tubes 49, which begins when the timer56 operates the valve 55.

In each of the embodiments the cross-sectional shape of the nozzles maybe varied from the circular, for example the cross section may be oval.In place of nozzles the front faces of the supply ducts 28 and 29 may beformed with arrays of slot- or slit-shaped apertures which are capableof generating streams of closely-packed, aerated particles forprojection towards the surface of the glass.

The invention produces thermally toughened sheets of glass with highvalues of central tensile stress and commensurate high values of surfacecompressive stress. The central tensile stress is an indication of thehigh strength of the toughened glass.

For example central tensile stresses in the range 114 MPa to 128 MPahave been produced in glass sheets of thickness in the range 6 mm to 12mm using the method of the invention.

Thinner glass sheets of thickness in the range 2 mm to 3 mm have beenproduced, using the invention, having a central tensile stress in therange 60 MPa to 92 MPa, as well as sheets of that thickness range havinga central tensile stress below 60 MPa, for example down to about 46 MPa.

Even thinner glass sheets can be thermally toughened to a high strengthby the method of the invention. For example toughened glass 1.1 mm thickhas been produced with a central tensile stress as high as 53 MPa. 9n

We claim:
 1. A method of thermally toughening glass in which the hotglass is quenched with a particulate material, characterised bygenerating an array of streams of aerated particles, the particles inwhich streams are sufficiently closely-packed so that each stream has avoidage fraction in the range 0.9 to 0.4 to enhance heat transfer awayfrom the glass by the particulate material in the toughening process,and projecting those streams of closely-packed particles towards theglass and regulating the stream velocity to ensure not only that eachstream of particles maintains its closely-packed integrity in itstrajectory towards the glass but also that the glass surfaces are notdamaged by the closely-packed particulate material.
 2. A method ofthermally toughening glass in which the hot glass is quenched with aparticulate material, characterised bygenerating an array of streams ofaerated particles by supplying aerated particulate material into saidstreams from a supply body of aerated particulate material, theparticles of which streams are sufficiently closely-packed to enhanceheat transfer away from the glass by the particulate material in thetoughening process; supplying additional gas into the aeratedparticulate material which supplies those streams; regulating thepressure of the additional gas supply to regulate the stream velocity ofeach of said streams; and projecting those streams towards the glasswith stream velocity which ensures that the integrity of each stream ispreserved in its trajectory towards the glass and that the glasssurfaces are not damaged by the particulate material.
 3. A method ofthermally toughening glass in which hot glass is quenched with aparticulate material, comprisinggenerating an array of aerated streamsof closely-packed particles; supplying additional gas into the aeratedparticles supplying those streams; and regulating the pressure of theadditional gas supply to regulate projection of the streams towards theglass with stream velocity which preserves the integrity of each streamin its trajectory towards the glass and is not detrimental to the glasssurface.
 4. A method according to claim 1 or 3, wherein each stream ofparticles has a voidage fraction in the range 0.76 to 0.4.
 5. A methodaccording to claim 1 or claim 3, wherein the component normal to theglass surface of the velocity of each stream of particles is at least 1m/s.
 6. A method according to claim 1 or claim 3, wherein the glass is aglass sheet which is substantially vertical and the streams of particlesare directed towards the surface of the substantially vertical sheet. 7.A method according to claim 1 or claim 3, wherein the hot glass is a hotglass sheet which is supported horizontally and arrays of streams ofparticles are projected upwardly and downwardly towards the surfaces ofthe sheet.
 8. A method according to claim 1 or claim 3, wherein saidarray of streams of particles is projected from an array of nozzleswhich communicate with a supply body of aerated particulate material. 9.A method according to claim 1 or claim 3, wherein the component normalto the glass surface of the velocity of each stream of particles is inthe range of about 1 to 5 m/s.
 10. A method of thermally toughening aglass sheet, comprising suspending a hot glass sheet, generating aplurality of streams of closely-packed, aerated particles each of whichstreams has a voidage fraction in the range 0.9 to 0.4, and projectingthose streams of closely-packed particles towards the surfaces of theglass sheet and regulating the stream velocity such that the componentnormal to the glass surface of the velocity of each stream is in therange of about 1 to 5 m/s to ensure not only that each stream ofparticles maintains its closely-packed integrity in its trajectorytowards the glass but also that the surfaces of the sheet are notdamaged by the closely-packed particulate material.
 11. A method ofthermally toughening a glass sheet, comprising:suspending a hot glasssheet; generating a plurality of streams of closely-packed, aeratedparticles by supplying aerated particulate material to form saidstreams; projecting said streams of particles from arrays of nozzleswhich communicate with a supply body comprising a falling supply of theparticulate material including entrained gas; supplying additional gasinto the falling supply of the particulate material adjacent thenozzles; and regulating the height of the supply body above the nozzlesand the pressure of the additional gas supply to regulate the velocityof projection of the streams from the nozzles towards the glass at avelocity which ensures that the integrity of each stream is preserved inits trajectory towards the glass surface.
 12. A method according toclaim 11, wherein the pressure in the aerated particulate materialadjacent to the entrances to the nozzles is regulated by maintaining apressure above the surface of the supply body.
 13. A method of thermallytoughening a glass sheet, comprising suspending a hot glass sheetbetween two vertical arrays of nozzles, supplying to each array ofnozzles a flow from a falling supply of aerated particulate material,projecting from said nozzles towards the surfaces of the glass aplurality of streams of closely-packed, aerated particles, and supplyingadditional gas into the flows adjacent the arrays of nozzles to projectthose streams at a velocity which ensures that the integrity of eachstream is preserved in its trajectory towards the glass.
 14. A methodaccording to claim 13, including switching a gas supply to each flow ata plurality of locations which are spaced apart vertically relative toeach other adjacent the nozzles to initiate projection of the streams ofparticles towards the next glass sheet to be toughened.
 15. A methodaccording to claim 14, including selectively timing the switching of gassupply to those locations, beginning with the lowermost location. 16.Apparatus for thermally toughening glass, comprisingmeans for containinga supply of aerated particulate material; gas supply means located insaid containing means for pressurizing said supply of aeratedparticulate material; means for generating from that supply in saidcontaining means an array of streams of closely-packed, aeratedparticles, and for projecting those streams towards a surface of theglass; and means for regulating the pressurizing of said supply ofaerated particulate material thereby regulating the voidage fraction ofeach stream in the range 0.9 to 0.4 and the velocity of projection ofeach stream with a stream velocity which ensures that the integrity ofeach stream is preserved in its trajectory towards the glass withoutdetriment to the surface quality of the glass.
 17. Apparatus forthermally toughening glass, comprising:a container for a supply body ofaerated particulate material; an array of nozzles communicating with thecontainer for projecting streams of closely-packed, aerated particlestowards a surface of the glass; gas supply means located in saidcontainer for generating in said container a pressure in said supplybody; and means for regulating gas supply to said gas supply means toproject said streams towards a surface of the glass at a velocity whichensures that the integrity of each stream is preserved in its trajectorytowards the glass and that the glass surface is not damaged by theparticulate material.
 18. Apparatus according to claim 17, wherein saidcontainer is a closed container for the supply body, with the array ofnozzles connected to one side of the container, and said gas supplymeans is connected to the top of the container to pressurise the spacein the container above the supply body.
 19. Apparatus according to claim18, characterised by two of said closed containers for two supply bodiesof aerated particulate material, each container having an array ofnozzles, which arrays of nozzles are positioned to define between then atreatment space for a hot glass sheet.
 20. Apparatus for thermallytoughening glass comprising:a container for a supply body of aeratedparticulate material, which container is a supply duct which isconnected to a supply vessel for containing a body of aeratedparticulate material and which supply vessel is positioned to provide aneffective head of pressure for supply of the particles; an array ofnozzles connected to the supply duct for projecting streams ofclosely-packed aerated particles towards a surface of the glass; gaspermeable means for gas supply located in the supply duct adjacent tothe entrances to the nozzles; and means for regulating gas supply tosaid gas permeable means to regulate the velocity of projection of thestreams from the nozzles.
 21. Apparatus according to claim 20, includingtwo of said supply ducts each with a vertical array of nozzles, whicharrays define between their outlet ends a vertical treatment space for asuspended glass sheet, and two of said supply vessels respectivelyconnected to said supply ducts.
 22. Apparatus according to claim 21,wherein individual air slides connect the supply vessels to therespective supply ducts to maintain the particulate material in anaerated state as it is supplied to the supply ducts.
 23. Apparatusaccording to claim 20, characterised by two of said supply ducts eachwith a horizontal array of nozzles, which arrays constitute upper andlower arrays of nozzles which point towards each other and definebetween them a horizontal treatment space for a glass sheet. 24.Apparatus according to claim 21, including a tank for collection of theparticulate material from the streams, collection chutes for particulatematerial mounted adjacent the tank to collect particulate material whichspills over the top edges of the tank, and recirculation conveyors whichlead from the collection chutes to the tops of the supply vessels torecirculate particulate material which spills from the tank.